1. Field of the Invention
This invention relates to a phase change optical recording medium and a method for overwriting such optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. For example, Specification (Ver. 1) for DVD-RAM which is a phase change optical recording disc was published in July, 1997. A phase change optical recording medium is a medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type use chalcogenide systems, such as Ge--Te system and a Ge--Sb--Te system, which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It has also recently been proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe.sub.2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) such that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single laser beam.
In the optical recording medium of phase change type, performance of the medium has been determined by whether C/N of sufficient level is realized at the linear velocity used, and whether sufficient erasability is achieved in the erasure. An excessively high erasability, however, adversely affects the performance of the medium. A high erasability means a high susceptibility of the recording layer to crystallization, when the recording layer is highly susceptible to crystallization, heat diffusion in the lateral direction during the formation of the rear end of the record mark results in the gradual cooling, and hence, recrystallization of the front end of the record mark. Such recrystallization results in the erasure of the front end of the record mark, and such erasure is referred herein as "self erasure". A medium of excessively high erasability, therefore, needs optimization in the erasability to prevent increase in jitter by such self erasure.
For example, JP-A 7176/1997 discloses a process for preventing self-erasure upon use of a medium adapted for high linear velocity at a low velocity. In this process, a pulse pattern is optimized in the pulse train of the recording laser beam depending on the linear velocity.
The inventors of the present invention, however, found that prevention of jitter increase by the optimization of the medium design is associated with a new problem. When the medium which has been designed to minimize the self-erasure at the linear velocity used is stored at a high temperature of approximately 60 to 80.degree. C. after the formation of the record marks and the record marks are overwritten with new record marks, the original record marks can not be sufficiently erased. When the erasure of the original record marks is insufficient, the newly recorded signals can not be normally read out. Jitters and errors are thereby greatly increased. The temperature inside optical disc drives and other media drives during their operation is as high as at least about 60.degree. C., and the medium is exposed to such high-temperature environment for a quite long time. Therefore, prevention of the decrease in the erasability by high-temperature storage is an urgent issue.
The leading edge and the trailing edge of the record marks can be separately evaluated for their jitter. The jitter at the leading edge is mainly caused by the self-erasure while the jitter at the trailing edge is mainly caused by the insufficient erasure. A reason for the increase of the jitter (total jitter) can be effectively analyzed by evaluating the jitter at each edge of the record marks.